6Superior canal dehiscence

A bony defect over the superior semicircular canal creates a "third window" that shunts acoustic energy away from the cochlea — and elevates the SP/AP ratio in a pattern that looks deceptively like Ménière's disease. Telling them apart is the diagnostic point of this module.

Lloyd Minor's group at Johns Hopkins first described superior canal dehiscence syndrome in 1998 as a constellation of sound- and pressure-induced vertigo, autophony, pulsatile tinnitus, low-frequency air–bone gap on audiometry, and a bony defect over the superior semicircular canal seen on high-resolution temporal bone CT.[1998] What looked at first like a vestibular curiosity turned out to be a clinical entity with a coherent biomechanical explanation — the dehiscence acts as a third mobile window on the inner ear, in addition to the oval and round windows — and a clear ECochG signature that has been validated across multiple cohorts.[2009, 2015, 2015, 2024]

FClinical presentation

The classical SCD syndrome is dominated by sound- and pressure-related symptoms that distinguish it from other vestibular disorders. ECochG is one component of the workup — alongside high-resolution temporal bone CT (the diagnostic gold standard) and cVEMP/oVEMP (the most sensitive vestibular indicators).

TThe third-window mechanism

In a healthy inner ear, acoustic energy delivered at the oval window propagates through the perilymph, deflects the basilar membrane, and exits via the round window. The pressure differential across the basilar membrane is what drives cochlear transduction. The oval and round windows are the only two compliant points in the bony labyrinth — every other surface is rigid bone — so essentially all acoustic energy contributes to basilar membrane deflection.

A dehiscence over the superior canal punches a third hole in the otic capsule. Now some fraction of the acoustic energy delivered at the oval window takes the path of least resistance through the dehiscence rather than through scala vestibuli toward the round window. This has three consequences:[2009, 2016]

1. Reduced air conduction at low frequencies— energy is lost to the shunt, producing the low-frequency air–bone gap on audiometry.
2. Enhanced bone conduction at low frequencies — the lowered impedance of the inner ear makes bone-conducted sound more efficient (and produces autophony).
3. Altered intracochlear pressure dynamics — the basilar membrane operating point shifts, the MET system reads from an asymmetric resting position, and the SP rises relative to the AP. This is mechanistically distinct from hydrops (no endolymph distension, no Reissner bulge) but produces a similar-looking elevated SP/AP ratio. Creighton et al. proposed that the dehiscence reduces perilymphpressure (hydrops ex vacuo) rather than raising endolymph pressure, which inverts the conventional Ménière's interpretation but yields the same recorded sign.[2016]

TMechanism explorer

Acoustic energy delivered at the oval window normally propagates through scala vestibuli and exits via the round window. A dehiscence over the superior semicircular canal opens a third compliant path — a fraction of the energy is shunted upward through the canal, the pressure differential across the basilar membrane shifts, and the SP rises relative to the AP.

Toggle between the intact otic capsule and the dehiscent state to see the acoustic-energy path change. With the canal sealed, all the energy delivered at the oval window propagates through scala vestibuli to the round window (amber). When the canal is dehiscent a fraction of the energy is shunted out through the third window (red), the intracochlear pressure dynamics shift, and the SP rises. Adams et al. (2015) report an SP/AP cutoff of 0.34 giving 92.3% sensitivity and 94% specificity for SCD when combined with clinical symptoms and CT findings.[2015]

FECochG findings in SCD

Three decades of clinical work have established a consistent ECochG signature, though absolute values vary with technique:

The non-correlation findings are clinically important. They mean the SP/AP ratio is a binary functional marker — present or absent — rather than a gauge of severity. You cannot use the ratio to predict whether a small radiographic dehiscence is producing significant symptoms; that question depends on the clinical history and VEMP findings.[2015]

TMénière's vs SCD on the same axes

Both Ménière's disease and SCD produce an elevated SP/AP ratio. The traces look similar enough that you would not be able to confidently distinguish them on ECochG alone. The diagnostic discrimination is in the audiogram, the VEMPs, the symptom pattern, and the CT — not in this curve.

CReversibility after surgical plugging

One of the most striking findings about SCD-related ECochG abnormalities is that they reverse after surgical plugging of the dehiscence. Adams's original 2015 cohort included 13 patients who underwent middle cranial fossa repair; five had postoperative ECochG and showed normalisation of SP/AP ratios alongside symptomatic improvement.[2015] This was extended by Creighton et al. (2016) who used intraoperative ECochG to monitor the SP/AP ratio in real time during the plugging procedure.[2016]

FClinical case

FTSelf-assessment